Default behavior is to check the releases API for the newest stable version, and prompt if one is available.

Use --stable to install or reinstall the latest stable version.

Use --nightly to install the latest built version of the master branch. While not recommended for production, nightly contains the latest and greatest, and should be stable enough for development and staging environments.

# Update CLI.
$ wp cli update
You have version 0.24.0. Would you like to update to 0.24.1? [y/n] y
Downloading from https://github.com/wp-cli/wp-cli/releases/download/v0.24.1/wp-cli-0.24.1.phar...
New version works. Proceeding to replace.
Success: Updated WP-CLI to 0.24.1.